Serum thyroxine, triiodothytonine and reverse triiodothyronine in early postnatal period in pig (preliminary communication).

The rapid increase in serum thyroxine (T4), triiodothyronine (T3) and reverse triiodothyronine (rT3) in the pig shortly birth presumably due to enhanced secretory activity of the thyroid gland was observed. During the 2nd and 3rd week of live T3 and T4 remained essentially unchanged but rT3 continued to rise to a high plateau level. This resulted in an increase in rT3/T4 ratio to about two and rT3/T3 raio to about one. A shift to rT3 production seems to be a reflection of an increased capacity of 5-iodothyronine deiodinase with age.